This document provides additional information about LEX and describes several LEX patterns, variables, functions, start conditions, and examples. It explains that LEX is used for text processing and scanner generation. It also describes pattern matching symbols, special directives like ECHO and REJECT, variables like yytext, and functions like yylex(), yyless(), and yywrap(). Examples are provided for multi-file scanning, counting character sequences, and generating HTML.

1. More on LEX

2. LEX
 In addition to scanner development, Lex/Flex is also used for
some other applications in system administration where text
processing is needed.

3. Patterns
 r? zero or one occurrence of r
 r{2,5} two to five occurrences of r
 r{2,} two or more occurrences of r
 r{4} exactly 4 occurrence of r
 0 a NUL character (ASCII code 0)
 123 the character with octal value 123
 x2a the character with hexadecimal value 2a

4. Patterns
 <s>r an r, but only in start condition s
 <s1,s2,s3>r same, but in any of start conditions s1, s2,
or s3
 <*>r an r in any start condition, even an
exclusive one.
 <<EOF>> an end-of-file
 <s1,s2><<EOF>> an end-of-file when in start condition s1
or s2

5. Special Directives
 ECHO copies yytext to the scanner's output.
 BEGIN followed by the name of a start condition places the
scanner in the corresponding start condition.
 REJECT directs the scanner to proceed on to the second best rule
which matched the input (or a prefix of the input). Multiple
REJECT's are allowed, each one finding the next best choice to
the currently active rule.

6. Example %{ int s=0;
%}
%%
she {s++; REJECT;}
he {s++}
%%
main() { yylex();
printf(“%dn",s);
return 0; }

7. Lex Variables
 yyin Of the type FILE*. This points to the current file
being parsed by the lexer.
 yyout Of the type FILE*. This points to the location where
the output of the lexer will be written. By default,
both yyin and yyout point to standard input and
output.
 yytext The text of the matched pattern is stored in this
variable (char*).
 yyleng Gives the length of the matched pattern.
 yylineno Provides current line number

8. Lex Functions
 yylex() The function that starts the analysis.
 yywrap() In the C program generated by the lex
specification file, when yylex() finished with the
first file, it calls yywrap(), which opens the next file,
and yylex() continues. When yywrap() has
exhausted all the command line arguments, it
returns 1, and yylex() returns with value 0.
 unput(c) puts the character c back onto the input stream. It
will be the next character scanned.
 input() reads the next character from the input stream

9. Lex functions- yyless(n)
 Returns all but the first n characters of the current token back
to the input stream, where they will be rescanned when the
scanner looks for the next match.
 yytext and yyleng are adjusted appropriately
 An argument of 0 to yyless will cause the entire current input
string to be scanned again.

10. For example, on the input "foobar" the following will write out
"foobarbar“
%% foobar ECHO; yyless(3);
[a-z]+ ECHO;

11. Lex functions- yymore()
 yymore() tells the scanner that the next time it matches a
rule, the corresponding token should be appended onto the
current value of yytext rather than replacing it.

12. For example, given the input "mega-kludge" the following will
write "mega-mega- kludge" to the output
%% mega- ECHO; yymore();
kludge ECHO;
First "mega-" is matched and echoed to the output. Then
"kludge" is matched, but the previous "mega-" is still
hanging around at the beginning of yytext so the ECHO for
the "kludge" rule will actually write "mega-kludge".

13. Start Conditions
 Flex provides a mechanism for conditionally activating rules.
 Any rule whose pattern is prefixed with "<sc>" will only be
active when the scanner is in the start condition named "sc".
<STRING>[^"]* { /* eat up the string body ... */ ... }
will be active only when the scanner is in the "STRING" start
condition
<INITIAL,STRING,QUOTE>. { /* handle an escape ... */ ... } will
be active only when the current start condition is either
"INITIAL", "STRING", or "QUOTE".

14.  A start condition is activated using the BEGIN action.
 Until the next BEGIN action is executed, rules with the given start
condition will be active and others are inactive.
 Start conditions declared using %s are inclusive, then rules with
no start conditions at all will also be active.
 Start conditions declared using %x are inclusive, then only rules
qualified with the start condition will be active.
 Exclusive start conditions make it easy to specify "mini-scanners"
which scan portions of the input that are syntactically different
from the rest (e.g., comments).

15. Begin
 BEGIN(0) returns to the original state where only the rules with no
start conditions are active.
 BEGIN(INITIAL) is equivalent to BEGIN(0).
int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
<SPECIAL>[a-z] {printf(“letter”);} %%

16. Example %s TABL REC DATA
%%
<INITIAL>"<table>" BEGIN TABL;
<TABL>"</table>" BEGIN INITIAL;
<TABL><tr> BEGIN REC;
<REC></tr> BEGIN TABL;
<REC><td> BEGIN DATA;
%%
<DATA></td> BEGIN REC; main()
<DATA>. ECHO; {
<DATA,REC,TABL>[ tn] ; yylex();
}

17. Line no in text file
%{
/*
*/
int lineno=1;
%}
line .*n
%%
{line} { printf("%5d %s",lineno++,yytext); }
%%
main(int argc, char*argv[])
{
extern FILE *yyin;
yyin=fopen(argv[1],"r");
yylex();
return 0;
}

18. A Lex program that finds a word from the given file and replaces
it with other word.

19. %{ {eol} {fprintf(fr,yytext);}
#include<stdio.h> . {fprintf(fr,yytext);}
#include<string.h> %%
FILE *ff,*fr; int main(int argc,char
char p[20],q[20],r[20],fname[20]; *argv[])
%} {
word [a-zA-Z]+ strcpy(fname,argv[1]);
%% strcpy(p,argv[2]);
{word} { strcpy(q,argv[3]);
if(strcmp(p,yytext)==0) ff=fopen(fname,"r+");
fprintf(fr,q); fr=fopen("rep.txt","w+");
else fprintf(fr,yytext); yyin=ff; yylex();
} return(0);
}

20. Replace Arabic Nos with Roman
%{
#include<stdio.h>
#include<ctype.h>
FILE *ff,*fr;
char fname[20];
int z;
%}
%%
1 {fprintf(fr,"I ");}
2|3|4 { z=atoi(yytext);
while(z!=0)
{
{fprintf(fr,"I "); z--;
}
}

21. 5 {fprintf(fr,"V ");}
6|7|8 { z=atoi(yytext);
{fprintf(fr,"V ");}
while(z!=5)
{
{fprintf(fr,"I "); z--;
}
}
9 {fprintf(fr,"IX ");}
10 {fprintf(fr,"X ");}
. {fprintf(fr,yytext);}
%%
int main(int argc,char *argv[])
{
strcpy(fname,argv[1]);
ff=fopen(fname,"r+");
fr=fopen("rep.txt","w+");
yyin=ff;
yylex();
return(0);
}

22. Date Format Change
%{
#include<stdio.h>
#include<string.h>
FILE *ff,*fr;
char fname[20];
int z;
%}
digit [0-9]
%%
{digit}{digit}""{digit}{digit}"" {yymore();}
[5-9][1-9] {fprintf(fr,"19"); fprintf(fr,yytext);}
[0-5]{digit} {fprintf(fr,"20"); fprintf(fr,yytext);}

23. %%
int main(int argc,char *argv[])
{
strcpy(fname,argv[1]);
ff=fopen(fname,"r+");
fr=fopen("rep.txt","w+");
yyin=ff;
yylex();
return(0);
}

24. The following lex specification file is used to generate a C
program which is used to generate a html file from a data file.
A0 = "Name"
B0 = "SSN"
C0 = "HW1"
D0 = "HW2”
…
Name SSN HW1 HW2
Scott Smith 123445678 82 44.2
Sam Sneed 999887777 92 84

25. %%
^A0 {printf("<table>n<tr>n");}
^A[09]* {printf("</tr>n<tr>n");}
^[BZ][09]* ;
[09]* {printf("<td> "); ECHO; printf("</td>n");}
[09]*"."[09]* {printf("<td> "); ECHO; printf("</td>n");}
"[^"]*" {printf("<td> "); yytext[yyleng1]= ' ';
printf("%s </td>n",yytext+1);}
= ;
[ n] ;
. ECHO;
%%
main() {
printf("<html>n");
yylex();
printf("</tr>n</table>n</html>n");
}

26. This lex specification file is used to generate a C program
which counts how many times a given alphabet is next to
another alphabet in the given input

27. %{
#include<stdlib.h>
int F[26][26];
%}
%%
[AZaz][AZaz]
{
F[toupper(yytext[0])65][toupper(yytext[1])65]
++;REJECT;
}
.|n ;
%%

28. main(int argc, char*argv[])
{
extern FILE *yyin;
int i,j;
for(i=0;i<26;i++)
for(j=0;j<26;j++)F[i][j]=0;
yyin=fopen(argv[1],"r");
yylex();
for(i=0;i<26;i++){
for(j=0;j<26;j++) printf("%d", F[i][j]);
printf("n");
}
return 0;
}

29. %{
#include <stdio.h>
#include <errno.h>
Yywrap() Example
int file_num;
int file_num_max;
char **files;
extern int errno;
%}
%%
(ftp|http)://[^ n<>"]* printf("%sn",yytext);
.|n ;
%%
int main(int argc, char *argv[]) {
file_num=1;
file_num_max = argc;
files = argv;
if ( argc > 1 ) {
if ( (yyin = fopen(argv[file_num],"r")) ==
0){
perror(argv[file_num]);
exit(1);

30. }
}
while( yylex() )
;
return 0;
}
int yywrap() {
fclose(yyin);
if ( ++file_num < file_num_max ) {
if ( (yyin = fopen(files[file_num],"r")) ==
0){
perror(files[file_num]);
exit(1);
}
return 0;
} else {
return 1;

31. A lex-based squid redirector
%x SKIP
%x COPY
%option always-interactive
%%
"http://www.debian.org/" |
"http://www.yahoo.com/" {
yytext[yyleng-1] = '0';
printf("%s.au/",yytext);
BEGIN COPY;
}
"http://"(www.)?"altavista.digital.com/" {
printf("http://www.altavista.yellowpages.com.au/");
BEGIN COPY;
}
"ftp://sunsite.anu.edu.au/pub/linux/" |
"ftp://sunsite.unc.edu/pub/Linux/" |

32. "ftp://ftp.funet.fi/pub/Linux/" {
printf("ftp://ftp.monash.edu.au/pub/linux/");
BEGIN COPY;
}
"http://weather/" {
printf("http://www.bom.gov.au/");
BEGIN COPY;
}
"http://bypass." { printf("http://"); BEGIN COPY;
}
"ftp://bypass." { printf("ftp://"); BEGIN COPY; }
<COPY>[^ n] ECHO;
<COPY>" " BEGIN SKIP;
. BEGIN SKIP;
<SKIP>. ;
<*>n { putchar('n'); fflush(stdout); BEGIN 0; }
%%

33. Write a LEX program to count the
number of palindrome string in a given
file and write them into a separate file.

34. Write a LEX program to read 3 Roman
numbers and output its equivalent
numbers and sum in decimal.
Ex:
Input: VI IX L
Output: 6 + 9 + 50 = 65

35. Write a LEX program to read an
expression in postfix form and
evaluate the value of the expression
Ex: 342+* is 18

36. Write a lex program to read a text file
and reverse all words that begin with
letter ‘a’ and end with letter ‘d’. Display
the longest of such words.

37. Write a LEX program to count number
of even and odd numbers. Output the
odd numbers to odd.txt and even
numbers to even.txt files. Also display
the largest odd and even numbers

38. Write a lex program to read a text file
and remove all words that begin with
letter ‘a’ and end with letter‘d’. Write
the remaining words to a separate file.

39. Write a lex program to read a text file
and do the following
Add 1 to all integers.
Add 0.5 to all fractional numbers.
Convert all words to uppercase words
Delete all alphanumeric words
Write resulting content to a separate
file.

40.  Encryption and decryption

41. Write a Lex program that converts a file
to "Pig latin."
Specifically, assume the file is a sequence
of words (groups of letters) separated by
whitespace. Every time you encounter a
word:
1. If the first letter is a consonant, move
it to the end of the word and then add
ay.
2. If the first letter is a vowel, just add
ay to the end of the word.
All nonletters are copied intact to the
output.

42. Thank yu…!
3 September 2012 42